1. Field
Implementations of the present disclosure generally relate to methods and apparatus for forming a film on a substrate. More particularly, implementations of the present disclosure relate to methods and apparatus for heteroepitaxial growth of crystalline films.
2. Description of the Related Art
Group III-V and Group IHV compounds are finding greater importance in the development and fabrication of a variety of semiconductor devices, such as, for example, light emitting diodes (LEDs), laser diodes (LDs) and logic circuit devices such as field effect transistors (FETs. In these devices, a plurality of semiconductor layers having different mixed crystal compositions are layered together to obtain intended optical and electrical characteristics.
Group III-V and Group II-IV compound films are generally formed by heteroepitaxy, a form of epitaxy. In epitaxy, a monocrystalline film is deposited on a monocrystalline substrate from gaseous or liquid precursors. During deposition, the substrate acts as a seed crystal, the deposited film takes on a lattice structure and orientation identical to those of the substrate. The deposited film is typically referred to as an epitaxial film or epitaxial layer. In heteroepitaxy, the epitaxial film and the substrate include different materials. Typically, substrates having similar properties as the target epitaxy film are used in heteroepitaxy to prevent deformation or cracking of the substrate and the epitaxy film during and after epitaxy. For example, gallium nitride (GaN) is formed by heteroepitaxy on sapphire, and aluminum gallium indium phosphide (AlGaInP) on gallium arsenide (GaAs).
In an attempt to reduce cost in forming group metal nitride based devices, metal nitride films are manufactured on larger and cheaper substrates. However, metal nitrides and silicon substrates have different properties, such as lattice constant and thermal expansion coefficient. The differences in properties introduce defects in the metal nitride film and even cause silicon substrates and nitride films to crack when growing high crystalline nitrides. For example, when growing gallium nitride material on a silicon substrate, mechanical stress and thermal stress are generated due to difference in lattice constants and thermal expansion coefficients causing the silicon substrate to bow and the gallium nitride film and even the silicon substrate to crack.
Epitaxial wafer materials are widely used as starting materials in semiconductor device fabrication. The presence of defects in such wafer materials can seriously affect the subsequent device performance. For example, GaN and its related compounds InGaN and AlGaN are widely used in the fabrication of short-wavelength semiconductor laser diodes. The performance of such laser diodes is seriously degraded by the presence of threading dislocations, which thread vertically through the epitaxial layers. Similar defects are found in other material systems, for example, when GaAs is grown on SiGe/Si. A reduced dislocation density on the epitaxial wafer materials is therefore desired.
One approach to reducing the defect density involves growing thick strain relaxed buffer (SRB) layers on the substrate to absorb that may be targeted for strain relaxation. Others have attempted to grow dissimilar materials that have a tendency to relax quickly on the substrate of choice, or otherwise grow thicker films that eventually relax due to accumulated strain. However, these processes often take a long time to complete and involve costly materials.
Therefore there is a need for methods for forming strain thinner relaxed buffer layers that reduce defect density while reducing processing times and costs.